GPs could be routinely testing patients for their genetic risk of cancer within five years following a landmark study.
The biggest investigation of its kind ever carried out has greatly multiplied the number of known genetic markers linked to breast, prostate and ovarian cancer.
Scientists compared the DNA of cancer patients with a similarly sized sample from the general population
In future, the results may make it possible to single out individuals with a potentially lethal hand of genetic cards using a simple saliva test.
They can then be monitored closely for the first signs of developing cancer, or - in the most high risk cases - be offered preventative treatment.
Experts believe the cheap and easy tests could be conducted in family doctors' surgeries.
Samples would initially be sent off to laboratory specialists, but eventually GPs could be doing the analysis themselves.
"We're on the verge of being able to use our knowledge of these genetic variations to develop tests that could complement breast cancer screening and take us a step closer to having an effective prostate cancer screening programme," said study author Professor Doug Easton, a Cancer Research UK scientist from Cambridge University.
Ovarian cancer tests are also envisaged, but these will take longer to develop after more research.
The international scientists compared the DNA of more than 100,000 cancer patients with a similarly sized sample from the general population.
They used microchip technology capable of identifying more than 200,000 genetic variants, some of which were suspected of being linked to cancer.
The analysis highlighted more than 80 inherited regions of the genetic code that increase an individual's risk of developing one of the three cancers.
Each variant - a one-letter change in the code called a single nucleotide polymorphism (SNP) - on its own has a small impact on cancer risk. But added together, large numbers of the variants greatly increase the chances of disease.
In the case of prostate cancer, men in the top 1% of the risk table are 4.7 times more likely than the average man to develop the disease.
Half of all men in this category who also have a strong family history of prostate cancer can expect to succumb to the illness during their lifetime.
Similarly just under a third of women in the 1% of those most at risk of breast cancer are likely to develop the disease. This excluded women carrying the well-known high risk genes BRCA1 and BRCA2.
For ovarian cancer, being in the top 1% having many of the the variants more than doubled the lifetime risk from 1.8% to 4%.
The variants found tended to affect the activity of genes rather than the kind of proteins they made.
A total of 23 new genetic risk factors for prostate cancer were discovered, increasing the known total to 78.
Of these, 16 were associated with particularly aggressive and life-threatening forms of the disease.
The number of SNPs linked to breast cancer was more than doubled to 49, while 11 new variants that raise the risk of ovarian cancer were found.
More than 1,000 scientists from 130 institutions in Europe and the US took part in the Collaborative Oncological Gene-environment Study (Cogs).
Their findings appear in 13 papers in five journals, Nature Genetics, Nature Communications, PLoS Genetics, the American Journal of Human Genetics and Human Molecular Genetics.
The chip used in the analysis cost just £30, and although the study relied on blood samples the same tests can easily be carried out on saliva.
This opens up the possibility of patients being tested for their genetic risk of cancer in GP surgeries.
Professor Ros Eeles, a leading member of the team from the Institute of Cancer Research in London, said: "Eventually it will be at the point of care because the person making the decision is very likely to be your GP.
"Although we're not there yet, I think we're going to get there fairly fast."
The SNP test could become widely available for both breast and prostate cancer within five years, she said.
Some women already known to have BRCA genes that greatly increase the risk of breast cancer may be tested in as little as 18 months.
Much more research is still needed to identify the genetic drivers of cancer, said the scientists.
In the case of all three diseases, about 60% of the genetic risk factors remained unexplained.
The researchers also stressed that lifestyle and environmental influences worked together with genes to raise the risk of cancer.
As well as helping to identify high and low risk individuals, the findings could pave the way to new treatments and highlight previously unknown aspects of cancer development.
It was hoped that some drugs already in use could be adapted to target the biological pathways uncovered.
Dr Harpal Kumar, chief executive of Cancer Research UK, which co-funded the research, said: "This groundbreaking international work highlights how complex cancer is.
"Hundreds, if not thousands of genes are likely to play a role in how cancers start. But by understanding why some people seem to be at greater risk of developing cancer we can look towards an era where we can identify them and take steps to reduce their chances of getting cancer or pick up the disease at its earliest stages."
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